Electrical apparatus, such as motors and turbine generators, occasionally overheat due to shorts or other malfunctions. The longer the overheating continues the more damage is done to the apparatus. A malfunction detected immediately may mean only a quick repair but if the overheating continues the entire machine may be damaged.
Large rotating electrical apparatus is usually cooled with a hydrogen gas stream. The organic compounds in the apparatus are first to be affected by the overheating and they decompose to form particles which enter the gas stream. Monitors then detect particles in the gas stream and sound a warning or shut down the apparatus when too many particles are detected.
Descriptions of such monitors and how they function may be found in U.S. Pat. No. 3,427,880 titled "Overheating Detector for Gas Cooled Electrical Machine" and in U.S. Pat. No. 3,573,460 titled "Ion Chamber for Submicron Particles." Another monitor, "The Condensation Nuclei Detector," is described by F. W. VanLuik, Jr. and R. E. Rippere, in an article titled "Condensation Nuclei, A New Technique for Gas Analysis," in Analytical Chemistry 34, 1617 (1962) and by G. F. Skala, in an article titled "A New Instrument for the Continuous Detection of Condensation Nuclei," in Analytical Chemistry 35,702 (1963).
As U.S. Pat. Nos. 3,427,880 and 3,807,218 suggest, special coatings may be applied to the apparatus which decompose and form detectable particles at a lower temperature than the usual organic compounds found in the apparatus. For example, those patents mention polyalphamethylstyrene, polystyrene, polymethyl methacrylate, and cellulose propionate which decompose to form particles at 230.degree. to 340.degree. C. Unfortunately, since these machines normally operate at about 50.degree. to 100.degree. C, they may be severely damaged by the time the temperature reaches 230.degree. to 340.degree. C.
Efforts to identify materials which will decompose to form detectable particles (i.e., thermoparticulate) at temperatures closer to the operating temperature of the machine have met with several difficulties. Many compounds, such as succinic acid, maleic acid, fumaric acid, and polyacrylic acid, do not decompose below 190.degree. C. Others, such as acetic acid, are liquids which boil and therefore are unsuitable. Some compounds, such as oxalic acid, decompose at a low temperature but the decomposition products do not include detectable particles. Compounds such as 1,2-diformylhydrazine have some of the desirable properties but cannot withstand several years operation at 80.degree. to 100.degree. C. A few compounds contain toxic or corrosive substances in their decomposition products which may render them unsuitable.